Field of the Invention
The present invention relates to a superconducting current lead, a superconducting current lead device, and a superconducting magnet device.
Description of the Related Art
When a superconducting applied device such as a superconducting magnet is active, the superconducting magnet needs to be cooled. As a cooling method, a method that immersing the magnet into refrigerant such as liquid helium and liquid nitrogen (i.e., an immersion cooling type) and a method utilizing heat conduction from a refrigerator or a refrigerant (i.e., a conduction cooling type) are known.
In order to generate a magnetic field by using a cooled superconducting magnet, the superconducting coil needs to be excited up. Therefore, a current lead is used in order to supply current to a superconducting magnet from a power source.
A current lead is constituted by a conductor and formed of a good conductor such as Cu with low electric resistance. A good conductor has large thermal conductivity, and thus, heat penetration from an exterior increases. In addition, since Joule heat is generated in the current lead, a cooling efficiency of a superconducting magnet deteriorates by the Joule heat.
The cooling efficiency directly influences the cooling cost (i.e., electricity usage in a case of a refrigerator). Therefore, in particular, in a case of a conduction cooling type using a refrigerator, a current lead using a superconductor instead of the good conductor described above may be applied.
When an oxide superconductor is used as the superconductor, since electric resistance is zero, theoretically, Joule heat is not generated. In addition, an oxide superconducting layer has low thermal conductivity since the layer is formed of ceramic. As a result, a desirable current lead in which heat penetration from an exterior can be suppressed and Joule heat at a lead portion is reduced can be structured.
Conventionally, as the current lead described above, a superconducting current lead including an oxide superconductor bulk has been used. However, the oxide superconductor bulk has relatively weak mechanical strength, and thus, the usage is limited due to its fragility. On the other hand, a superconducting current lead using a Bi-based oxide superconducting wire, which is one kind of a high-temperature superconductor, is partially commercialized.
However, the Bi-based high-temperature superconductor requires a lot of wires since the critical current in a high-temperature magnetic field decreases due to a relation between the critical current and a magnetic field characteristic. In addition, the Bi-based high-temperature superconducting wire has a structure such that the high-temperature superconducting layer is covered with an Ag coating layer which is a good conductor. Therefore, an area ratio of the Ag coating layer is large, and a large amount of heat is introduced from an exterior due to heat conduction.
There is a current lead using an Y-based high-temperature superconducting wire having a favorable relation between the critical current and a magnetic field characteristic compared to that of the Bi-based high-temperature superconductor. As an example of application of such current lead, a structure of a current lead in order to suppress drift current and to prevent the current from not returning from a superconducting state (i.e., quench) is described in Japanese Unexamined Patent Application, First Publication No. 2009-230913.
Moreover, a structure of a current lead which has a non-inductive winding shape in order to reduce a decrease of the critical current due to a magnetic field in a superconducting wire is described in Japanese Unexamined Patent Application, First Publication No. 2009-230912.
In the structure of the current lead using the Y-based high-temperature superconducting wire having a favorable relation between the critical current and a magnetic field characteristic compared to that of the Bi-based high-temperature superconductor, when the current lead is applied to the superconducting magnet, the magnetic field is always applied to the current lead. In the superconducting wire, the critical current decreases based on the strength and the applied angle of the applied magnetic field. However, magnetic-field applied angle dependence in the Y-based high-temperature superconducting wire may vary based on the various fabricating methods, and sufficiently detailed data thereof is not available. Moreover, as a required characteristic of the current lead, it is not preferable that the critical current decreases with respect to a magnetic field in only a certain direction. As a measure, multiple Y-based high-temperature superconducting wires may be arranged with a small angle. However, when a lot of superconducting wires are provided, since a sum of areas of a stabilization layer in each of the superconducting wires increases, it is assumed that heat penetration from an exterior through the stabilization layer is not negligible.